A significant environmental problem for many hydroelectric facilities is the water quality of turbine discharges. The primary water quality problem is often low dissolved oxygen (DO) levels which occur seasonally. During the summer months, thermal stratification of the reservoir produces a surface layer of less dense, warm water with relatively high dissolved oxygen and a lower, relatively cold oxygen-depleted layer resulting from the decay of organic material on the reservoir bottom. Because hydraulic turbines draw their discharges from the lower level, this can create water quality problems downstream of the facility.
Relicensing and rehabilitation of a hydroelectric facility offer an opportunity to address environmental and industrial development concerns over dissolved oxygen levels and other water quality regulations which affect hydropower releases. Rehabilitating an existing hydroelectric facility may, as explained in co-pending U.S. patent application Ser. No. 08/733,366 filed Oct. 17, 1996 to the same assignee, include replacement of the runner. Replacing an existing runner with a new runner having integral passages, and providing air through the integral passages, can be used to enhance dissolved oxygen levels in the water without disturbing the existing foundation of the facility.
To improve water quality in prior art turbine installations having shrouded runners, it is known to supply air through the foundation to the shroud spaces adjacent the runner crown and band. In particular, it is known to admit air through the discharge ring to the annular chamber between the band and the discharge ring. It is also known to supply air through the head cover to the annular chamber between the crown and the head cover.
Providing air admission to these annular chambers has heretofore been done, however, for two reasons. First, the aeration of these chambers has been used to quiet pressure pulsations being caused by part load or overload vortices within the draft tube. Second, the aeration has been used to expel water from these chambers, while maintaining the air within the chambers, to reduce the disc friction losses and thereby increase operating efficiency of the turbine. In other words, in this second case the aeration devices are designed to separate the gas from the water rather than to have the gas entrained by the water as the water exits the chambers. Accordingly, it appears desirable to develop new ways to improve the water quality of hydropower releases, and thereby increase the survivability of fish downstream of such turbine installations.